The development of the antrioventricular canal (A-V canal) of embryonic chick hearts (35 to 120 h) was studied by morphological and electrophysiological techniques. The earliest identification of the A-V canal action potentials was at 45 to 49 h of incubation coinciding with the atrium formation and the appearance of its corresponding distinctive action potential. At that time the first atrioventricular delay was recorded and its action potentials showed a low rate of rise, particularly at the initiation of the upstroke and a long duration. The conduction velocity was the lowest in the A-V canal. As the development proceeded from 45 to 120 h, the cardiac cells of the A-V canal showed scanty membrane to membrane contacts and large intercellular spaces filled with abundant extracellular matrix components, in striking contrast to the paucity of these components in the atrium and ventricle. The morphological and electrophysiological characteristics described for the A-V canal cells, could help to explain their slow conduction properties and the atrioventricular delay.